Improve error reporting for type signatures

[ghc-hetmet.git] / compiler / typecheck / TcMType.lhs

diff --git a/compiler/typecheck/TcMType.lhs b/compiler/typecheck/TcMType.lhs
index 12f3a73..55b16d9 100644 (file)
--- a/compiler/typecheck/TcMType.lhs
+++ b/compiler/typecheck/TcMType.lhs
@@ -422,7 +422,7 @@ zonkTcTyVarsAndFV tyvars = mappM zonkTcTyVar tyvars `thenM` \ tys ->
                           returnM (tyVarsOfTypes tys)
 
 zonkTcTyVar :: TcTyVar -> TcM TcType
-zonkTcTyVar tyvar = ASSERT( isTcTyVar tyvar )
+zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
                    zonk_tc_tyvar (\ tv -> returnM (TyVarTy tv)) tyvar
 \end{code}
 
@@ -1137,7 +1137,7 @@ check_inst_head dflags clas tys
     (first_ty : _) = tys
 
     head_shape_msg = parens (text "The instance type must be of form (T a1 ... an)" $$
-                            text "where T is not a synonym, and a1 ... an are distinct type variables")
+                            text "where T is not a synonym, and a1 ... an are distinct type *variables*")
 
        -- For now, I only allow tau-types (not polytypes) in 
        -- the head of an instance decl.  
@@ -1183,7 +1183,11 @@ checkValidInstance tyvars theta clas inst_tys
                         undecidableMsg])
 \end{code}
 
-Termination test: each assertion in the context satisfies
+Termination test: the so-called "Paterson conditions" (see Section 5 of
+"Understanding functionsl dependencies via Constraint Handling Rules, 
+JFP Jan 2007).
+
+We check that each assertion in the context satisfies:
  (1) no variable has more occurrences in the assertion than in the head, and
  (2) the assertion has fewer constructors and variables (taken together
      and counting repetitions) than the head.